When people think of human impacts on the environment, they often think in terms of total population and population growth. The scale of our activities depends on population, consumption and the resource or pollution impact of technologies; all three of these factors are steadily increasing.
Urbanisation
Rapid urban growth can bring environmental problems for cities. With many cities growing at 4 to 5% a year, provision of clean water, sewage, electricity and roads can rarely keep up with population growth. Lack of sewage treatment leads to water pollution, eutrophication and biodiversity loss in rivers and around outlets. Water demand may lower river and groundwater levels. The International Decade for Drinking Water Supply and Sanitation (1980-90) documented the growth of those without clean water in urban Africa, rising from 28 million to 31 million; those without safe sanitation rose from 38 million to 47 million.
We have transformed half the world's land for our own uses -- around 11% each for farming and forestry, 26% for pasture, and another 2 to 3% for housing, industry, services and transport. In most parts of the world, cultivated land has not expanded in line with population growth, decreasing the amount of farmland per person. The area per person has declined slowly in developed countries, from 0.65 hectares in 1965, to 0.51 hectares 30 years later. In developing countries, where population growth is faster, the area per person fell from 0.3 to 0.19 hectares over this same period.
Freshwater
Freshwater is crucial for survival, health, agriculture, industry, comfort and leisure. But freshwater sources are limited -- there is only so much to go around: the larger the population, the less there is for each person.
In 1995, some 436 million people were already suffering water scarcity or stress, causing severe development problems. There are conflicts among farmers and between rural and urban needs, and heightening tensions between countries dependent on the same resources, such as with India and Bangladesh.
The UN's 1996 population projection has estimated that, by 2050, the projected number of people suffering water stress or scarcity will have risen to 4 billion approximately.
Biodiversity
Most ecologists believe that human activities are causing mass extinction. Since 1600, 484 animal and 654 plant species are known to have become extinct through human actions. The total extinction of a species is drastic and irreversible, but local extinction is also serious, and far more common. The Global Biodiversity Assessment listed the major threats to biodiversity as habitat loss, fragmentation and degradation, due to the need for land for farms, dwellings, industry, services, transport and leisure. Of those species that are threatened, habitat loss affects 44% of bird species, 55% of fishes, 68% of reptiles, and 75% of mammals.
Population density is closely linked with most forms of habitat loss. A sample of non-desert countries where wildlife habitat loss has been estimated showed that the percentage loss tends to be highest where population density is highest. The top 20% of countries, ranked in terms of habitat loss, had lost an average of 85% of the original wildlife habitat. Their average population density was 189 people per square kilometre. The 20%, with lowest population density, had lost an average of 41% of their wildlife habitat -- and their average population density was only 29 people per square kilometre.
Forests
We lose forests at the rate of 112 million hectares each decade, an area twice the size of Kenya or France. Highly populated countries such as India and China have almost come to the end of their period of deforestation and have begun to reverse forest loss. Some of the fastest rates of deforestation are found in middle-income developing countries with strong commercial logging interests (Indonesia 2.4%, the Philippines 3.5%, and Thailand 2.6%).
A number of studies have found a strong correlation between population density and deforestation rates on national levels. A recent report by the United Nations Population Fund estimated an average loss of 1.8% of forests per year between 1980-90, where the population density was 89 people per square kilometre. Areas with slower deforestation tend to have lower population density; where there are just 34 people per square kilometre, the deforestation rate was only 0.5%.
Coastal environment
High percentages of human population and activity are located on or near coasts. Coastal areas have always been important for trade, transport and defence, containing some of the densest concentrations of human population and activities today. Nearly two-fifths of the world's populations live within 150 kilometres of a coastline. A recent assessment found that over half the world's coastlines are at risk from coastal development, with over one- third at high risk. Nearly three quarters of the world's marine protected areas are similarly threatened. In addition, human activities over vast inland areas impact coasts and coastal waters. Much of the water pollution and sediment eroded from whole watersheds is transported to the sea.
Mangroves
Mangroves cover an estimated 18 million hectares of the earth's tropical coastlines, around one quarter of the total. Mangroves host unique species, and are important nurseries for commercial marine species.
It is estimated that around half of all tropical mangroves have been destroyed. The Philippines, Puerto Rico, Kenya and Liberia have lost over 70%. Major pressures are cutting for fuel wood and timber; habitat conversion for coastal development or aquaculture (often shrimp farming); and damming of rivers which alters water salinity. Other direct and indirect causes of these pressures include: population growth, tourism and resource consumption in and around coastal areas.
Coral reefs
The world has an estimated 255, 000 square kilometres of near-surface coral reefs, constituting one of the richest resources of biodiversity on the planet. A recent study estimated that 58% of the world's reefs are threatened by human activity, almost half of these seriously so. In Southeast Asia, which has very high levels of coral and fish diversity, more than 80% are potentially at risk.
The threats to coral reefs are many: over-fishing, pushing fish stocks below their maximum sustainable yield; destructive fishing practices; and extraction. Water pollution from industry, sewage, fertiliser, and sediment eroded from deforested or badly farmed areas, all wash into the sea, reducing light levels and physically smothering corals.
Marine environment
Oceans make up seven-tenths of the planet's surface, and we use an estimated 8% of their total primary productivity. Yet we have fished up to the limits or beyond, altering the ecology of a vast range of marine species.
Assessments from 1999 found that 44% of major fish stocks have already been exploited to their maximum sustainable yield. Another 16% are over-fished, meaning future catches will fall unless remedial action is taken.
Pollution from oil spillages, runoff and rivers includes sewage, industrial effluents, fertilisers, pesticides and herbicides. Air pollution is the source of one-third of marine pollutants.
There are now around 50 known "dead zones" with no or low oxygen. Most of these have appeared over the last half-century, and are blamed on excessive influx of nitrogen and phosphorus from farming and sewage. The dead zone in the Gulf of Mexico is 4,144 square kilometres, doubling in size since 1993.
We need to voluntarily limit our growth, and promote contraceptive use, before nature controls our population for us with famines, droughts and plagues. Our children's future depends on us.
How people preserve or abuse the environment largely determines whether living standards improve or deteriorate. Population growth, urban expansion, and resource exploitation do not bode well for the future. Without practicing sustainable development, humanity faces a deteriorating environment and may even invite ecological disaster.
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